1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (hereinafter, referred to as an OFDM) system, and more particularly to an apparatus and a method for compensating for residual frequency offset in an OFDM system.
2. Description of the Related Art
The present OFDM technology is a promising communication technology applied to various fields such as Digital Audio Broadcastings (DABs), Digital Video Broadcastings (DVBs), or high-rate wireless local area networks (LANs).
Such an OFDM system can transmit a great amount of information through a bandwidth defined by applying a high-order quadrature amplitude modulation (hereinafter, referred to as a QAM) such as a QPSK, a 16 QAM, a 64 QAM, or a 256 QAM to a subcarrier. However, the current OFDM system is sensitive to a frequency offset due to a doppler spread or a frequency disagreement of local frequency oscillators between a transmitter and a receiver resulting in transmission errors.
Particularly, the frequency offset in the OFDM system interferes with the orthogonality between subcarriers and causes an intercarrier interference (hereinafter, referred to as an ICI), thereby causing an amplitude reduction and a phase rotation of a signal.
Even after a fine frequency is estimated within +/−0.5 of the frequency offset of a subcarrier interval in the OFDM system, a residual frequency offset and a phase offset still remain. The residual frequency offset causes the ICI between subcarriers and the deterioration of a bit error rate (hereinafter, referred to as a BER) performance.
A DVB-T system will be described as an example. Even though approximate frequency synchronization that compensates for frequency values corresponding to subcarrier intervals and a fine frequency offset compensation that compensates for a frequency within the subcarrier interval are performed, the phase rotation due to the residual frequency offset and phase noise still remains. As a result, the BER performance is deteriorated by the ICI.
A conventional OFDM system such as a DVB-T compensates for a frequency offset by performing the capture and tracing of the frequency offset by periodically inserting a plurality of pilot bits into an OFDM symbol. Such a method exhibits good BER performance by performing a synchronization modulation. However, since the pilot information is inserted into a data transmission segment, data transmission efficiency is inevitably reduced. Compensation for a carrier offset and a phase offset is often performed on data by a decision-directed (hereinafter, referred to as a DD) method.
However, such a DD method cannot compensate for unfavorable circumstances such as a Rayleigh fading channel in a DVB-T system because the frequency capture range is narrow.
In the conventional DD method, a phase difference between a received signal and a hard-decided representative signal for each symbol is obtained by the following Equation 1. A phase error ψ(m) for an mth OFDM symbol using the DD method is expressed as:
                              ψ          ⁡                      (            m            )                          =                              1            N                    ⁢                                    ∑                              k                =                0                                            N                -                1                                      ⁢                          Im              ⁡                              [                                                                            Y                      m                                        ⁡                                          (                      k                      )                                                                                                                          X                        ^                                            m                                        ⁡                                          (                      k                      )                                                                      ]                                                                        (        1        )            where, Im represents an imaginary part of [x], Ym(k) is a kth subcarrier of an fast fourier transform (hereinafter, referred to as an FFT) demodulated mth symbol, and {circumflex over (X)}m(k) represents a hard-decided, estimated transmission signal of the FFT demodulated signal Ym(k).
Although the phase error is detected through Equation 1, frequency capture performance of the DD method deteriorates as an order ‘M’ in an Multi-level-QAM (hereinafter, referred to as an M-QAM) increases. This is because the number of M-QAM signals which are not located on a diagonal axis increase when the order ‘M’ increases. Accordingly, the DD method is not suitable for tracking the residual frequency offset in a high-order QAM.